This invention relates to methods for producing connecting rods having a thermally sprayed bearing layer and to connecting rods produced thereby.
Conventional connecting rods now in use, especially for internal combustion engines, are so-called cut or cracked connecting rods in which the large connecting rod eye that surrounds the crankshaft is cut or cracked to open it. As a rule, the small connecting rod eye does not need to be opened since it is connected to the piston by a straight bolt.
Depending upon the load applied to the bearing, connecting rod eyes are made with a variety of bearing shells providing the friction surface. In particular, supporting shell materials used in bearing shells are as a rule made of C 10 steel according to DIN 17210 or SAE 1010. Depending upon the particular design and application, the bearing shells may be cold hardened. The actual bearing surface layer, which may, for example, be white metal, leaded bronze, light metal, spatter coatings or the like depending upon the expected bearing load, may be applied to the supporting shell material. The bearing shells may be three-component, two-component or solid single component bearing shells. The shells are assembled to the connecting rod eye with an initial stress SO that the bearing shells have a satisfactory, firm seat upon assembly.
Bearing shells not only constitute a substantial cost factor, but also complicate production and are a potential source of error. For example, the insertion of a bearing shell or bearing shell half may be overlooked in assembly, resulting in considerable engine damage.
Accordingly, it is an object of the present invention to provide a connecting rod with a thermally sprayed bearing layer which overcomes disadvantages of the prior art.
Another object of the present invention is to provide a connecting rod bearing having a high strength bearing layer.
These and other objects of the invention are attained by roughening the bearing surface of a connecting rod eye by sandblasting it with successively different particle size distributions before depositing a bearing layer by thermally spraying a bearing material on the surface.
Thus, according to the invention, a bearing shell is no longer inserted into a connecting rod eye, either the large eye, or alternatively into both connecting rod eyes. Instead, a bearing layer is applied directly to the connecting rod eye by thermal spraying such as by plasma spraying. Specifically, the connecting rods made in this way are used in an internal combustion engine to connect the crankshaft to the pistons. To increase the adhesive pull strength of the bearing layer, the connecting rod eye to be coated can be roughened preferably by abrasive sandblasting to be carried with sands of successively different particle-size distributions. To increase the quantity of residual oil on the bring layer, the layer can have a groove and/or is microporous.
The various particle-size distribution curves that are advantageously used according to the invention are also known as mesh size classes, where according to the invention the classes from 16 mesh to 230 mesh i.e., 16, 18, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 100, 120, 140, 170, 200 and 230 in particular are used. The mesh count specifies the number of holes per square inch, i.e., per 6.45 cm2 in the screen used. For example, the individual particle-size distribution curves, including tolerances and sieve wire sizes, can be obtained from the Handbook of Chemistry and Physics, 64th Edition, 1983/1984, CRC Press Inc., Florida, p. F-114.
It has been determined by thermally coating a connecting rod eye that the usable bearing layers, which are made of aluminum bronze for example, have a high residual layer stress which in particular is increased by the fact that the material is deposited with a low porosity. Consequently, it is clear that measures to increase the bonding strength of the bearing layer are necessary. This is advantageously achieved in accordance with the invention in that the connecting rod eye is sandblasted with successively different grain sizes including at least one fine and at least one coarse grain size distribution.
Operationally reliable connecting rod bearings require a wear-resistant design and construction in order to transmit the bearing forces reliably and at permissible operating temperatures. Wear resistance is always provided when the sliding surfaces are separated from each other by a lubricating film such as an oil film that is capable of bearing a load. Such a lubricating film is maintained in friction bearings by a slightly eccentric shaft mounting. With this arrangement, the rotating shaft has a pumping effect that feeds the lubricant such as motor oil into the eccentric bearing slot, and oil pressure is built up at the convergent surfaces of the bearing slot. In other words, the lubricant is pressed into the narrowest cross section of the space between the bearing surfaces. This results in a condition called xe2x80x9cinterfacial lubricationxe2x80x9d in which there is interfacial friction of the bearing material on the journal when the rotary motion of the shaft begins or is very slow. An increase in rotation speed causes the oil film to assist in supporting the journal even though a coherent oil film has not yet been built up. This is a condition called xe2x80x9cmixed friction,xe2x80x9d i.e., simultaneous interfacial and floating friction. This condition exists chiefly when an engine is being started and stopped. An additional increase in the speed of rotation causes the development of the hydrodynamically supporting lubricating film layer having a thickness equal to half of the bearing clearance resulting in a condition called xe2x80x9cfloating friction.xe2x80x9d With floating friction the bearing clearance is usually equal to about 15 xcexcm to 60 xcexcm.
When the surface of a connecting rod eye such as the large eye is plasma coated with an appropriate bearing material having micropores and/or a groove according to the invention, a high oil retention volume is obtained within the connecting rod bearing. As a result, friction and hence wear of the parts moving against each other, in particular during interfacial lubrication and mixed friction, are reduced. The oil retention volume is distinctly improved by the provision of at least one groove in the connecting rod bearing surface, the groove or grooves being preferably in the form of circumferential grooves. For an additional increase in oil retention capacity, the grooves are left at least largely unfinished, i.e., with their raw surface structure as produced.
In every bearing condition, regardless of the rotation speed and oil pressure, the microporous but pressure-stable structure of the sliding bearing surface, which preferably is machined, and the unfinished groove having a very rough surface structure cause a certain portion of oil to be stored in the bearing. This makes it possible for the bearing to pass through the conditions of interfacial and mixed friction more quickly even at a low crankshaft speed and hence reach the condition of nearly wear-free hydrodynamic lubrication rapidly. In other words, engine bearing performance characteristics during starting and slow-down of the engine are substantially improved so that higher bearing loads are possible with bearings having the same dimensions.
The invention has the following advantages: The bearing shells which were formerly customary in connecting rods to provide the sliding bearing surface are eliminated since, according to the invention, the bearing layer is applied directly to the connecting rod eye surface and not to an extra element inserted in the connecting rod eye. Hence, the necessity for assembly of bearing shells in the eye is also eliminated. Elimination of the bearing shells and of machining of the sliding bearing shell surface according to the invention results in a reduction of dimensional tolerance variations. There are three tolerances in conventional connecting rod bearings, the first tolerance being that of the crankshaft dimensions, while the second tolerance is determined by the dimensions of the bearing shells providing the sliding layer in the connecting rod and the third tolerance is determined by the dimensions of the connecting rod eye into which the bearing shell is inserted. As a result of coating and finishing of the bearing layer directly on the connecting rod eye surface according to the invention, the third tolerance is eliminated. In addition, according to the invention a greater thickness of connecting rod material is provided in the bolt region surrounding the eye since the bearing layer applied according to the invention is thinner than a bearing shell. This permits higher loads to be applied to the bearing of a connecting rod having the same external dimensions. The dimension of the third tolerance, i.e., the dimension of the connecting rod eye, may be sized very roughly in the present invention, since the eye is covered by the subsequent coating which is partially removed during formation of the sliding surface by, for example, fine spindling, to provide the second tolerance dimension.
According to the invention, the bearing layer is sprayed on so that it has a certain degree of porosity, at least at the bearing surface. This porosity is obtained by providing the bearing layer with micropores that are formed, for example, by oxide inclusions which are removed during finishing of the surface of the layer. The porosity of the bearing surface area is preferably between about 0.2% and about 6% and desirably between about 0.5% and about 4%. In addition, the micropores are preferably not interconnected, so that the pore volume of the micropores is constituted predominantly by closed pores. These micropores, which form a hydrodynamic micropressure chamber lubricating system, are opened up by, for example, the cutting operation of surface trimming, for example fine spindling of the bearing surface. As a result, the pores in the sliding bearing surface act as oil retention chambers so that, during startup or slowdown of the engine and at the beginning or termination of rotation of the crankshaft, a sufficient volume of oil is still available, presumably by adhesion of the motor oil in the micropores, for sliding film formation, i.e., floating friction, even though the oil pressure in the friction bearing has dropped or has not yet been built up. The duration of mixed friction operation during startup and slowdown is substantially shortened by the micropressure chamber system provided by the invention. The emergency operating characteristics of the bearing are also substantially improved without requiring the introduction of additional sliding bearing materials such as lead, tin or nickel alloys or the like. Advantageously, both the total pore area and the pore volume are determined during thermal spraying of the bearing materials according to the bearing load to be applied to the sliding surface. The major portion of the pore volume is constituted by pores of a size preferably in the range from 0.2 xcexcm to 250 xcexcm, desirably in a range from 1 xcexcm to 50 xcexcm.
The usual sequence of steps for the production of a bearing layer according to the invention is as follows: First the surface of the substrate to be coated, for example the large connecting rod eye, is cleaned, and especially freed from grease. This is carried out by applying superheated stearn, for example. Then, the surface of the substrate is sandblasted, for example with Al2O3 powder. SiO2 or SiC alternatively are usable. The discharge pressure is preferably about 3 to 8 bar and desirably about 4 to 6 bar, and the sandblasting is preferably carried out with successively different Al2O3 grain size ranges. In this connection, an increasing grain size is advantageously used, i.e., sandblasting is done first with a finer grain and subsequently with a coarser grain. Most preferably, at least three different grain size ranges are used. In this case, the grain sizes usually used are in a 15 to 250 mesh range, with 80 mesh and smaller, preferably 100 mesh to 230 mesh for the fine grain. For the medium-size grain, a size of 100 mesh or greater, preferably up to 40 mesh and desirably 80 mesh to 45 mesh, is used. For the coarse grain, a grain size of 45 mesh or greater and in particular 30 mesh or greater, preferably up to 16 mesh, i.e., 600 xcexcm to 1.18 mm standard screen size, is used.
Using grains of different size ranges in sandblasting results in good surface roughness, even in the region of a notch, for example a breaking notch or a groove, producing a surface structure with an average peal-to-valley height Ra of approx. 5 xcexcm to 10 xcexcm and in particular 6.5 xcexcm to 8 xcexcm in the smooth region, with an Rz of approx. 5 xcexcm to 60 xcexcm and in particular 42 xcexcm to 54 xcexcm. Such roughnesses produced by sandblasting result in especially good adhesive strength of the thermally sprayed layer on the connecting rod.
Sandblasting is followed by plasma coating, for example using an AlCuFe (aluminum bronze) alloy powder. The thermally sprayed bearing layer preferably is produced with progressively increasing porosity, a layer having low porosity, for example, xe2x89xa62%, in particular xe2x89xa61%, being produced first. For the layer, a powder grain size of 38 xcexcm (400 mesh), for example, is suitable. This first layer is produced with a thickness of approx. 100 xcexcm to 300 xcexcm, in particular 200 xcexcm to 250 xcexcm. Then a second layer having a porosity of approx. 2% to 6%, in particular 2.5% to 4%, is produced on the first layer by using a powder grain size of approx. 63 xcexcm (230 mesh), for example. In this case, the powder grain size is chosen so that at least 40% by weight, preferably at least 50% by weight, of the powder is smaller than or equal to the specified screen size, and preferably at least 70% by weight, and desirably at least 80%, by weight falls within the next standard screen size, and preferably at least 90% by weight falls within twice the specified screen size, i.e., half the mesh size.
The layer thickness is preferably in the range from 150 xcexcm to 800 xcexcm, desirably in the range from 200 xcexcm to 500 xcexcm. When only one layer is formed, it is preferably in the range from 100 xcexcm to 600 xcexcm, desirably in the range from 200 xcexcm to 400 xcexcm, thick.
It has been shown in accordance with the invention that the residual layer stress increases with decreasing porosity, giving rise to the danger of separation of the layer from the connecting rod eye. The present invention overcomes this in particular by special sandblasting steps, which produce an especially high bonding strength of the layer on the connecting rod eye. In addition, separation of the layer resulting from excessive residual layer stress is prevented in accordance with the invention by increasing the deposition temperature of the layer. This can be produced, for example, by increasing the voltage or current to a plasma burner. In this way, the layer is distributed better on the surface of the connecting rod eye, and better adhesion is achieved. This happens advantageously only to the extent that the applied layer has a certain small porosity as is described above. On the other hand, increasing the application temperature also brings with it the risk of annealing the substrate, in other words the substrate can be overheated. This is particularly problematic with ferrous materials. This problem in turn is counteracted according to the invention by depositing only a lower layer at the high application temperature. Further layers are then deposited at lower temperatures, and also with a higher porosity. If such higher porosity is unsuitable for the desired bearing material that does not present a problem if this more porous layer is removed by subsequent machining according to the invention. In this case, the more porous layer or the layer applied at lower temperatures merely serves to build up the substrate for subsequent mechanical machining.
Application of the bearing layers preferably is effected in one operation, i.e., the coating sequence is not interrupted. To accomplish this, automatic powder and/or parameter adaptation is effected in order to obtain different porosities in the layers.
The lower bearing layer preferably covers the out-of-roundness which is typically about 30 xcexcm to 150 xcexcm that is produced for example by the cracking of the connecting rod upon breaking open of the bearing shell, and, to obtain a sufficient thickness for later finishing, the lower layer may be covered with one or more additional and, particularly, more porous, layers that can be applied to the bearing material of the connecting rod in a similar fashion. The upper layer or layers in turn can be removed without any major problem all the way down to the first slightly porous layer to produce the actual sliding bearing surface. This procedure results in an adjustment of the internal stress of the bearing layer, slight annealing of the connecting rod and high adhesive strengths of the sliding bearing layer, which commonly are xe2x89xa720 N/mm2 and particular xe2x89xa725 N/mm2. Adhesive strengths above 28 N/mm2 are possible with this procedure. A layer hardness of, for example, approx. 185 HV0.3 can be obtained using aluminum bronze for the bearing layer.
For more highly loaded sliding bearing layers in the connecting rod, particularly the large connecting rod eye, as are found especially in diesel engines, it is advantageous to increase the residual oil volume within the sliding bearing layer. This is effected according to the invention by increasing the oil retention volume at engine standstill or at an engine speed below idling speed by the providing one or more oil grooves in the sliding bearing layer. In this way, the region of mixed friction is traversed in a shorter time upon engine startup or slowdown. Such shorter time is a result of longer maintenance or more rapid formation of the sliding film, which results from adhesion of the motor oil in the micropores and the grooves or grooves in the surface facing the crankshaft. The introduction of at least one groove in the bearing layer, in particular in a bearing with a somewhat porous surface, results in the lubricating film being maintained for a longer time even though the oil pressure of the oil pump has already dropped or is just being built up. The effect of grooves is especially advantageous when the grooves extend predominantly circumferentially, i.e., completely around the inner bearing surface. In this way each groove is self-contained, i.e., is an endless groove and, in particular, an annular endless groove. The groove advantageously is V-shaped or trapezoidal in cross section, the sides of the groove extending at an angle to each other of preferably 30xc2x0 to 80xc2x0 and desirably 45xc2x0 to 60xc2x0. A groove depth in the range of 0.2 mm to 1 mm and preferably 0.4 mm to 0.6 mm is suitable. The number of grooves depends mainly on the width of the friction bearing, the supporting portion of the bearing, and the required additional oil retention volume. The specified groove shape and depth preferably refer to the groove dimensions before application of a thermally sprayed coating. Plasma spraying is desirably used for applying the thermally sprayed coating.
Preferably after the formation of one or more grooves in the surface of the eye, the surface is coated with the bearing layer, in particular with the composite bearing layer having variable porosity which is described above. In subsequent finishing of the bearing surface the grooves preferably are finished only slightly or not at all since the rough plasma-applied layer structure produces especially good oil retention in the grooves.
In the application of the bearing layer, a metal powder is thermally sprayed and metal alloys are preferably used. In addition, a mixture of a variety of metals and preferably metal alloys is used. Such a mix may, for example, be a mixture of aluminum and tin, the mixture advantageously being obtained by mixing of the individual metal components in powder form.
The selection of the metal powder to be thermally sprayed depends, among other things, on the specific spray parameters and can easily be determined by a person skilled in the art by a series of tests. Advantageously bronzes, especially aluminum bronze (aluminum/tin), but alternatively copper bronze as well as metal-soft material layers and/or metal-solid lubricant layers, are employed as bearing materials. The soft materials used are, for example, soft metals, such as lead, which are distributed in a harder metal, such as an alloyed aluminum, for example, aluminum/copper/magnesium/chromium. However, other soft materials, such as, for example, fluorocarbon polymers, e.g., polytetrafluoroethylene, may alternatively be used. Compounds such as molybdenum disulfide, boron nitrite and graphite, for example, are suitable as solid lubricants.
The bearing material is preferably sprayed in excess to produce a layer thicker than the desired thickness which is then mechanically finished. Honing is used for finishing, fine spindling in particular being suitable. In honing, 20 xcexcm to 300 xcexcm and preferably 50 xcexcm to 200 xcexcm of the bearing material preferably are removed. In fine spindling, the excess to be removed advantageously amounts to 50 xcexcm to 1,000 xcexcm and preferably 100 xcexcm to 500 xcexcm.
After removal of the excess, the finished bearing layer preferably has a thickness of 150 xcexcm to 800 xcexcm and desirably 200 xcexcm to 500 xcexcm. Such a layer is considerably thinner than conventional bearing shells, which have a thickness in the range of about 2.5 mm. This means that either the connecting rod material surrounding the eye can be thicker, permitting higher loading, or that the material surrounding the eye can have the same thickness and the connecting rod can be produced with a lower weight.
According to the invention, the connecting rod eye, which is coated with the bearing material by thermal spraying, is preferably opened only after application of the bearing material. This procedure is especially preferred when the connecting rod eye is broken open by cracking. In this process, the inside of the connecting rod eye is provided with a notch at each of the desired rupture locations. The notches preferably are made by a laser, for example an FK laser which is moved through the connecting rod eye at an angle to the bearing surface of approx. 45xc2x0. The laser power used is preferably 5 kW to 10 kW. The notch advantageously has a width of 0.3 mm to 0.8 mm and a depth of 0.2 mm to 0.7 mm. The rupture location usually is roughly centered in the connecting rod eye. Alternatively, the notch may also be produced by eroding or by a broach, for example, by ramming.
In notching and breaking the connecting rod eye, the sequence of operations preferably is effected so that first the connecting rod eye is notched, for example, with a tool, a laser or by eroding, then the bearing material is applied by thermal spraying, and subsequently, breaking is effected. This procedure eliminates or minimizes the slot that would otherwise exist at the rupture location or between the individual bearing parts. At a high engine load such a slot promotes detachment of the oil film. Better lubricating properties are thus obtained by notching before coating of the bearing material. Under some circumstances, breaking may take place before coating of the bearing material, which is broken again after coating, preferably without notching.
When the connecting rod eye is opened by being cut, this preferably takes place before application of the bearing layer by thermal spraying. In this procedure, the connecting rod eye is cut apart and the resulting surfaces of the remaining connecting rod and the connecting rod cap are individually broached smooth. The parts are then reassembled, provided with bolt bores and threads and bolted together. Preferably, a notch is also introduced at the parting plane of the surfaces in the connecting rod eye after it has been bolted back together again and before coating the surface of the eye with the bearing material. The connecting rod eye is preferably broken open again to break the bearing layer. If this break is too uneven, the bearing layer itself must be notched.
The honing or spindling treatment of the bearing layer described above is effected especially advantageously only after breaking of the connecting rod eye. In this way, any burrs produced upon rupture of the bearing layer at the rupture edge are removed simultaneously with removal of excess bearing material.
In the connecting rod produced according to the invention, an oil channel may if desired be introduced into the connecting rod eye. The oil channel is preferably bored into the connecting rod eye only after application of the bearing layer and in particular after its finishing. In this connection, the oil channel may alternatively be bored through the connecting rod to the facing connecting rod eye.
According to the invention, the large planar side surfaces of the connecting rod are preferably finished only after application of the bearing layer. In this case the finishing is preferably carried out by grinding of the planar surfaces.
In the method according to the invention, coating is preferably preceded by a step in which the connecting rod eye is roughened. This roughening is preferably effected by sandblasting, but blasting with a liquid under high pressure is alternatively possible. In this connection, the material of the eye such as C 70 steel, is preferably roughened to produce an average peak-to-valley height Ra of 4 to 30 xcexcm, desirably 8 xcexcm to 12 xcexcm. Such average peak-to-valley height values produce especially good adhesion of the bearing material to the material of the connecting rod eye.
For advantageous finishing of the bearing layer in the connecting rod eye, at least one of the plane side surfaces of the connecting rod eye is covered with a template that has an opening in the region of the eye. The template opening should be approximately the same size as the eye, so that, on the one hand, the coating of the bearing surface is not affected by the presence of the template and, on the other hand, coating of the bearing material on the planar side surfaces in the region of the connecting rod eye is largely avoided. If only one planar surface of a connecting rod is covered by the template, the other planar surface preferably lies on a pallet that also has an opening like that of the template in the region of the connecting rod eye.
The connecting rod eyes of several individual connecting rods are preferably coated in one operation according to the invention. To this end, a plurality of connecting rods, preferably two to ten and desirably four to eight are positioned against each other so that the connecting rod eyes to be coated form a continuous cylindrical opening. For this purpose, a holder into which the connecting rods can be inserted with their eyes aligned may be provided on a special pallet. The connecting rods which are coated at the same time in this process preferably are kept as a group so that they can subsequently be installed in the same internal combustion engine. All connecting rods for the same internal combustion engine preferably are coated together by such superposition. When this is not structurally possible because of the large number of cylinders, for example twelve cylinders, at least the connecting rods of one row of cylinders, for example, six cylinders in a V-12 engine, are superimposed for coating. This procedure ensures that connecting rods of like quality are installed in the same internal combustion engine.
In an especially preferred method, a gas stream is passed through the connecting rod eye during thermal spraying, particularly, when a plurality of superimposed connecting rods is coated. Air that is conditioned and purified provides a particularly suitable gas stream. The gas stream should desirably be free of grease and moisture and, insofar as possible, having a temperature maintained within a range of approx. 20xc2x0 C. The gas stream preferably has a flow velocity, i.e., a downward flow rate, of about 3 m/s to 15 m/s and desirably 5 m/s to 8 m/s. Any overspray produced during thermal spraying is blown off by the gas stream.
Thermal spraying of the bearing material is preferably effected by using a rotating spray nozzle that is driven in a rotating manner through the connecting rod eye preferably starting from a location above the connecting rod eye. An especially uniform coating in a connecting rod eye is obtained by using such a spray nozzle. In coating the connecting rod eye according to the invention, the spray nozzle is advanced through the eye at a rate of preferably 0.5 mm/s to 20 mm/s, desirably 2 mm/s to 8 mm/s.
It is especially preferred that a plurality of layers, in particular four to thirty layers of the bearing material are applied to the connecting rod eye when the bearing layer is formed in the eye. The successive layers preferably are applied in opposite directions, which is effective to improve the layer quality. This is carried out by operating the spray nozzle to coat the surface of the connecting rod eye while running in and then running out while rotating that spray nozzle preferably in the same direction of rotation.
The connecting rods are coated with the bearing material in mass production according to the invention. In this case it is advantageous if at least a few connecting rods in each series are measured. In this connection, at least the peak-to-valley height Ra and/or the uniformity of distribution of the bearing material when a mixture is used, should be measured. Especially preferably, measuring of the connecting rod is effected in a nondestructive manner.
The features and procedures mentioned above and in the following description are equally applicable to the method and to the connecting rods made according to the invention.